1. Field of the Invention
The present invention relates to a loading and retraction apparatus for magnetic disc storage systems and, more particularly, to a loading and retraction apparatus which permits separation of function between load/retract and flying and which provides for a symmetrical lift force in a space saving design.
2. Description of the Prior Art
Magnetic disc storage systems are widely used to provide large volumes of relatively low cost computer-accessible memory or storage. A typical disc storage device has a number of discs coated with a suitable magnetic material mounted for rotation on a common spindle and a set of transducer heads carried in pairs on elongated supports for insertion between adjacent discs, the heads of each pair facing in opposite directions to engage opposite faces of adjacent discs. The support structure is coupled to a positioner motor, the positioner motor typically including a coil mounted within a magnetic field for linear movement and oriented relative to the discs to move the heads radially over the disc surfaces to thereby enable the heads to be positioned over any annular track on the surfaces. In normal operation, the positioner motor, in response to control signals from the computer, positions the transducer heads radially for recording data signals on or retrieving data signals from a preselected one of a set of concentric recording tracks on the discs.
As the density at which digital information is recorded on a magnetic recording surface is increased, the gap between the recording head and the magnetic recording surface must be decreased. The smaller the gap and the closer the magnetic head is positioned with respect to the recording surface, the more difficult it becomes to control the mechanical tolerances of the structure mounting the recording head. To overcome these mechanical difficulties, magnetic recording heads are placed in head assemblies adapted for floating on a thin film of air created by the laminar air flow due to the rotation of the recording surface. Modern magnetic disc drives incorporate rigid substrate discs, the surfaces of which are polished to a high finish so that the head can reliably fly on the air bearing. Systems are presently being designed wherein the heads fly above the disc recording surfaces at heights of less than 20 microinches.
In such systems, when the recording medium rotates, the laminar air flow causes the head assembly to be forced away from the medium. Therefore, some urging means, such as a spring, must be provided to overcome this air flow and counterbalance the head assembly, keeping it as close to the recording medium as possible. Furthermore, floating magnetic recording head assemblies are often mounted in gimbal mounting devices in order to allow the angle and position of the magnetic recording head assembly to conform to the air bearing.
The head suspension is generally a six-degree-of-freedom system. These six degrees are rotation and translation about two orthogonal axes (roll and pitch) parallel to the recording surface and the axis normal thereto. A typical mount has a very low spring rate for rotation of the head about any axis parallel to the recording surface. The spring rate for translation along an axis normal to the recording surface must be controlled quite closely to maintain the proper head-to-surface clearance. On the other hand, the head should be mounted so as to have very high spring rates for translation of the head parallel to the recording surface and in rotation about an axis normal to it.
The most effective apparatus used heretofore for achieving the desired result employs a gimbal sheet formed from a single, thin, approximately square, piece of resilient material, such as steel, for attaching a transducer head to a head arm. The head arm is a cantilevered member which is as rigid as possible to prevent any appreciable deflection of it during operation. The periphery of the gimbal sheet is attached at mounting points on its opposite edges to a side of the cantilevered head arm end so as to be positioned generally parallel to an adjacent recording surface. The head itself is attached to the center of the gimbal sheet.
Typical gimbal sheets and head arms are disclosed in U.S. Pat. No. 3,896,495 to Beecroft and U.S. Pat. No. 4,206,489 to Manzke et al. In these patents, as in virtually all other known mechanisms, a bias is applied to the gimbal sheet in such a way that it normally retracts the head assembly from the disc surface. This bias force is overcome by a separate load force mechanism when the head is in its flying condition. This requires the head assembly to be lifted off the disc surface by a retract force applied to the center of force of the air bearing. Any attitude control (pitch and roll of the air bearing) will be supplied by bias induced in the gimbal sheet. Since the gimbal sheets presently dealt with are very flimsy, any attitude controlling forces will be very small.
In other words, when the retract force is provided by a bias in the gimbal sheet, the retract force is limited to the force that can be developed by the structure of the gimbal sheet itself. Furthermore, when the gimbal sheet is biased, it may effect its attitude and this would be highly undesirable. Accordingly, the most desirable situation would be to permit separation of the functions between load and retract and flying. Thus, it would be desirable to eliminate all bias forces from the gimbal sheet and make the gimbal sheet totally flat. The load force for flying should be totally independent from the retract force.
Another problem encountered in the prior art results from the fact that prior art mechanisms for ramp loading magnetic flying heads onto a rotating magnetic disc having typically employed a ramp mechanism located on the outside edges of the head support arm. This has been thought to be convenient since the ramp can interact with an externally mounted cam surface to provide the load/unload action. The Beecroft and Manzke et al patents depict this type of external cam and follower ramp design. Vertical motion, required to lift the head from the disc surface, is provided by this combination.
Several problems are present in this implementation of ramp loading. In the device of Manzke et al, the external ramp and cam is provided on one side only of the cantilevered arm so that the action of lifting the ramp follower causes a twisting torque to be applied to the structure which will cause a pitch angle to be transmitted to the air bearing head. This pitch variation can be a troublesome variable which affects reliable head load/retract. Depending upon whether a particular arm has a head that is flying up or one that is flying down, the pitch can be positive or negative. With multiple head systems, one ends up with multiple different versions of twisting which causes pitch variations in the head and significant problems in manufacture to optimize the design around multiple different operating points.
To solve this problem, the Beecroft patent shows a symmetrical, twin ramp design with ramps on both sides of the cantilevered arm. This is done to overcome the problem previously mentioned since no twisting is emparted to the structure by the symmetrical lift. The problem, however, is that the arm width is now increased by the addition of two external cam surfaces. Another problem is in aligning the two cam surfaces exactly so that both cams touch the head arm at the same time in order to avoid twisting of the arm (as occurs with a single ramp/cam surface). The recent trend in the industry toward higher storage capacity in smaller packages makes space saving a prime design constraint and virtually eliminates from practical modern systems the additional width which results from two external cam surfaces.